Air suspension systems are one of the known suspension systems. An air suspension system includes an air compressor, an air suspension body, and a plurality of valves. In the air suspension system, the valves determine flow passages. The air compressor pressurizes and supplies air to the air suspension body to raise vehicle height, and the air is discharged from the air suspension body to lower the vehicle height.
Electric instruments installed in a vehicle are equipped with various types of failure detectors (see Patent Literature 1, for example). The Patent Literature 1 discloses a system including current source circuits for detecting failures, and current detection devices for detecting the current of the circuits. The system makes a comparison with proper two reference currents and distinguishes between a normal operation, a disconnection failure, and a ground fault failure.